1. Field of the Invention
The present invention relates to an ignition system having an ignition coil for an internal combustion engine.
2. Description of the Related Art
In a so-called distributor-less ignition system in which an ignition coil and a spark plug are integrally combined, a spark may be generated at the spark plug when current is supplied to the primary coil of the ignition coil at a timing other than the ignition timing. Such a spark causes abnormal vibration of an engine.
Such a troublesome spark can be prevented if a diode is connected between terminals of the primary and secondary coils so that the current induced in the secondary coil can flow to the primary coil, because the direction of the induced voltage (on-voltage) causing the troublesome spark is opposite to the direction of the regular ignition voltage. The troublesome spark can be also prevented if a diode is inserted in series with the spark gap of the spark plug to cut spark current flowing in the direction opposite to the regular ignition spark current. An additional spark gap connected in series with the spark plug gap can also prevent the troublesome spark.
However, such a diode to be used in a high tension circuit for the above purpose is expensive, and the space in the ignition coil to install the diode is limited. The additional spark gap requires an additional space and parts such as terminal members. The additional spark gap also increases the energy loss.
U.S. Pat. No. 4,653,460 discloses an ignition system using a zener diode connected in the high tention circuit of the ignition coil to improve the above problem. However, the zener diode is substituted for the regular diode only to pass the backward current when the on-voltage becomes higher than the breakdown voltage that is 2 K to 4 K volts. This zener diode is still expensive and bulky, because such zener diode consumes large energy and must have a large heat dissipation surface.